Joining of bright metals such as gold, copper, aluminium, platinum and silver by laser welding in the near infrared spectrum (800 nm to 2500 nm) presents a challenge, as the surface of bright metals are highly reflective with poor absorbance. To overcome the surface reflectivity and initiate coupling of the laser's energy into the metal surface, it is necessary to use laser beams with high power densities.
The function of the beam on a bright material approximates a discreet function with a very narrow operating window from beam hold-off (reflection) and absorption. At first the surface reflects substantially all of the laser light. However, once the surface reflectivity is overcome by sufficient laser intensity, a melt of the surface is initiated. The reflectivity then almost immediately transitions from its original highly reflective condition of more than 80% reflectivity to a lower value, which for some metals, can be less than 50% reflectivity. This causes the melt pool on the surface to grow extremely rapidly. It is consequently very difficult to control.
The challenge is increased when welding thin and low mass sections. Such high power densities are often detrimental, leading to over penetration of the laser beam and resulting unreliable joints. Conversely, if near infrared lasers are operated at lower power densities with beam intensities at or just above the absorbance limits, then this generally results in weak or absent welds as a result of inconsistent and random coupling of the laser beam to the metal surface.
The present preferred method of laser welding of copper and other bright metals such as gold and silver, involves the use of lasers that emit at visible green wavelengths. The most common lasers are frequency doubled 1064 nm lasers that emit at 532 nm. This is because the reflectivity of bright metals is usually significantly lower at 532 nm than at near infrared wavelengths. The laser joining of bright metals with such lasers produces welds that are repeatable and consistent but at the cost of efficiency, complexity, and costs associated with frequency doubling. In some applications, it is necessary to combine a laser emitting at 532 nm with a second laser at 1064 nm in order to increase efficiency and productivity. Such dual wavelength systems require closed loop monitoring of the laser welding process using sophisticated beam monitoring and real time analysis in order to analyze and tailor the structure of the weld. Such diagnostic devices use video analysis of the back reflected light and the weld pool characteristics in order to provide feedback to the laser controller. These systems are complex and expensive.
The use of green lasers has been adopted to perform weld joints of bright metals without specifically addressing the application of joining dissimilar metals. Conventional welding of dissimilar metals relies on specific control of the dilution of the metals at the interface and resulting thermal conditions to minimize mixing of the dissimilar metals which results in so-called intermetallics in the joint. A large intermetallic region is prone to fracture from stresses acting on the joint and the fracture propagates through the entire joint until failure.
Laser welding with continuous wave and pulsed lasers is well known, with either a continuous weld front, or overlapping spot welds in which the weld forms a continuous seam. Defects in the materials caused by the welding process create weaknesses, and are unacceptable in the majority of applications. Pulsed welds are typically formed using microsecond and millisecond pulses, generating melt which resolidifies to form the weld. When welding dissimilar materials, the weld interface can contain intermetallics, which are a compound formed from the two materials being joined, and are typically brittle and undesirable in nature, and the weld can therefore break along this intermetallic layer.
There is a need for a simpler solution for joining bright and dissimilar metals and alloys without problems caused in the joint interface. The method should be able to produce consistent and predictive results on each joint. The resulting weld should have no reliability issues associated with intermetallics.
There is a need for a method for an apparatus and method for laser welding that avoids the aforementioned problems.